1. Field of the Invention
This invention relates to an improved method of preparing esters of rosin. In particular, the improvement of this invention lies in reacting rosin and a polyhydric alcohol in the presence of phosphinic acid (H.sub.3 PO.sub.2) and a phenol sulfide compound, as combined catalyst, to reduce the reaction time for rosin pentaerythritol ester formation and to provide a rosin ester of improved color and oxidation stability.
2. Description of the Prior Art
Rosin is mainly a mixture of C.sub.20, fused-ring, monocarboxylic acids, typified by levopimaric and abietic acids, both of which are susceptible to numerous chemical transformations. The rosins to which this invention relates include gum rosin, wood rosin, and tall oil rosin or the rosin acids contained therein as for example, abietic acid, pimaric acid, sapinic acid, etc.
The natural separation and gradual conversion of some of the hydrophilic components of sap and related plant fluids from the cambium layer of a tree into increasingly hydrophobic solids are the generic process of forming diverse gums, resins and waxes. The oleoresin intermediate in this process is typified in pine gum, which flows from hacks on the trunks of southern yellow pine in southeastern United States, in France, and in other countries. Pine gum contains about 80% (gum) rosin and about 20% turpentine.
Resinification from oleoresin can result from either natural evaporation of oil from an extrudate or slow collection in ducts in sapwood and heartwood. Pinus stumps are valuable enough to be harvested, chipped, and extracted with hexane or higher-boiling paraffins to yield wood rosin, wood turpentine, and other terpene-related compounds by fractional distillation. In the kraft, i.e., sulfate, pulping process for making paper, pinewood is digested with alkali producing crude tall oil and crude sulfate turpentine as by-products. Fractionation of the crude tall oil yields tall oil rosin and fatty acids.
The chemical transformation of gum, wood, and tall oil rosin which relates to this invention is esterification. The beneficial product characteristics provided by rosin esterification for various applications have led to the development of many esterification procedures, particularly treatments with polyhydric alcohols. U.S. Pat. Nos. 2,369,125, 2,590,910 and 2,572,086 teach rosin esterification with glycerol and pentaerythritol, among other polyhydric alcohols, usually preceded by a rosin disproportionation step.
U.S. Pat. No. 3,780,012 teaches pretreating tall oil rosin with paraformaldehyde followed by distillation prior to the esterification reaction to achieve product color improvement. U.S. Pat. No. 3,780,013 teaches the incremental addition of a phenol sulfide compound during tall oil rosin pentaerythritol esterification. The color of the product of these procedures was claimed to be an M on the U.S.D.A. scale. The rosin color standards referred to in this invention are U.S.D.A. standards varying from X, the lightest, to D, the darkest color. The scale of colors is designated as X, WW, WG, N, M, K, I, H, G, F, E, and D. Also, due to the light color of the rosin ester products of the invention process, among X color rosins, the designations of X-A, X-B and X-C indicate lighter than X color, with X-C being the lighter color.
U.S. Pat. No. 4,172,070 teaches employing arylsulfonic acid in place of the traditional basic esterification catalysts, such as calcium oxide, to reduce the time for tall oil rosin-pentaerythritol esterification to obtain a rosin ester of improved oxygen stability, color and melting point. This work is confounded, however, by the unusually large amount of pentaerythritol used (35% equivalent excess) which by itself would markedly increase the rate of acid number drop. Products with Ring and Ball melting points of 77.degree. C. to 86.5.degree. C. were obtained. Normal commercial pentaerythritol esters of rosins have Ring and Ball melting between 95.degree. C. and 105.degree. C.
The object of this invention is to provide a novel method of preparing esters of rosin. It is a further object of this invention to employ a catalyst which accelerates the rosin esterification reaction rate to result in a reduced reaction time. It is a still further object of this invention to permit a reduction in the amount of polyhydric alcohol employed in the reaction, resulting in reduced cost and higher, more desirable melting points, i.e., from 95.degree. C. to 105.degree. C. It is a specific object of this invention to provide a method of preparing esters of rosin exhibiting a color equivalent to or lighter than the starting rosin.